It is known to provide a traveling web, e.g. a fabric at the time of manufacture with coding elements in the form of coding traces which extend transversely of the direction of travel of the web, parallel to one another in spaced relationship in the direction of travel. The coding tracings can be markings intended to be optically sensed or tracings detected by other sensor means, for example magnetic sensors. The coding may be applied at the time of manufacture and can identify different patterns, regions of the fabricated web, or the like.
Reading heads for such codes on code carriers can have a row of sensors responsive to the coding elements and the length of the row, which can extend transversely of the direction of travel, can be large and the mutual spacing of the sensors along the row can be small with respect to the maximum length of the coding elements or traces which can be of different length to convey the coded information. An evaluating circuit can be provided for the sensor signals.
In an earlier conventional device utilizing these principles, the total length of the sensor row defines the width of the reading region within which the coding can be detected and read. This reading width is greater than the maximum trace length of the coding so that the coding can be evaluated independently of its exact position in the reading region. As a consequence, there is no need to provide exact positioning of the code carrier as it travels by the reading region. Since the coding will always lie in the reading region the coding traces will not give rise to errors of the kind which has troubled earlier systems. Furthermore, errors along the length of a coding trace are eliminated.
For evaluating the sensor signal, this earlier system requires, independently from the reading width and from the total number of sensors in the row, only as many signal processing channels as will correspond to sensors along a coating trace of maximal length.
With this limitation in the signal processing channels, the signals from those sensors which correspond to the length of a coating trace are delivered to the signal processing channels.
In U.S. Pat. No. 3,790,756, the output signals of all signals after digitalizing, are fed to a shift register which is read out into the signal processing channels. The signal shifting in the register is effected until a first sensor signal appears at the register location corresponding to the first signal processing channel. This method has been found to have the disadvantage that, for each sensor, a respective circuit is required for digitalizing the sensor signal and by the need for shift registers with register locations for all of the sensors. With a large number of sensors this could be achieved only at high cost. Another drawback was the sensitivity of the device to noise or spurious signals which could appear at the sensors without having been generated by coding traces and could contribute errors in the read out of the signal processing channels with the result that false signal processing an signal evaluation can occur.